In many fluid handling systems, considerable precautions are taken to prevent leakage of fluid from the system to the ambient environment. In addition to the loss of the fluid itself, fluid leakage may result in explosions, fires, environmental contamination, and/or increased maintenance costs. Concerns about fluid leakage are particularly pronounced when the fluid being handled is corrosive or toxic.
One of the most likely locations for leakage in a fluid handling system is at the joint between the system components. In a valved piping system, one of the most troublesome leak locations is the interface between a valve and an adjacent system component, such as a pipe flange. In order to enhance sealing and minimize fluid leakage, a gasket is almost always interposed between adjacent system components.
When the fluid in the handling system is at an extreme temperature or pressure or when the fluid is corrosive or toxic, the most preferred and efficient type of gasket is a spiral wound type. A spiral wound gasket consists of preformed metal strips wound in a spiral with a selected filler material interposed and laminated between the metal strips. The filler material may be selected to meet the specific requirements of the system and the handled fluid. Common filler materials include asbestos materials, graphite and polytetrafluoroethylene (PTFE).
A valve seat is commonly secured relative to a valve body by a seat retainer, which retainer is securely fastened to the valve body to compressingly engage the interposed valve seat. By far the most prevalent method of fastening a seat retainer to a valve body is through the employment of screws which extend through screw holes in the retainer and are threadably received by bores in the valve body.
The existence of screw holes in a gasket face of a retainer poses significant limitations upon the effectiveness of a spiral wound gasket. Spiral wound gaskets tend to collapse into the voids created by the screw holes and do not adequately seal the voids. As a result, external leak paths for the fluid are created. Furthermore, the collapse of one or more spirals of a spiral wound gasket into a screw hole void has a domino effect upon the adjacent spirals. Thus, a leak path is created not only at the screw hole location, but also at adjacent areas.
The sealing band width of a spiral wound gasket is frequently less than the width of the retainer face and, for some valve sizes, it is possible to avoid the above mentioned problems by locating the screw holes either radially inside or radially outside this sealing band. However, locating the screw holes inside the sealing band decreases the area of the fluid flow path for many valves, reducing fluid flow through the valve and making the valve less efficient. Locating the screw holes outside the sealing band results in totally unsealed leakpaths through the screw holes.
Several prior art attempts have been made to eliminate retainer screw holes and to provide a full uninterrupted retainer face suitable for spiral wound gaskets. In one prior art design, the retainer is provided with a plurality of spaced lugs extending radially outward from the outer periphery of the retainer. These lugs have screw holes for receiving screws to hold the retainer to the valve body. Although the screw holes of such designs inherently have voids, just as the more conventional screw fastened retainers, the screw holes are located radially outside the retainer-gasket interface.
Unfortunately, such radially extending lug designs have several limitations. First of all, the valve seat closure member exerts a considerable force upon the valve seat. The seat, in turn, transmits this force to the retainer. Thus, when the retainer is fastened to the valve body only about the outer periphery of the retainer, the retainer becomes a relatively lengthy moment arm and is subject to excessive flexure. In order to avoid excessive flexure, such retainers must be relatively thick (have a relatively large axial dimension). As a result, standard valves having such retainers will not meet industry standards for end to end dimensions. Additionally, retainers of this prior art design cannot be recessed and, in effect, produce a valve with a split body having a potential leak path to the external environment around the entire circumference of the fluid flow path.
Another prior art approach is discussed in U.S. Pat. No. 4,399,833 to Holtgraver. The valve body discussed in the Holtgraver patent has lugs circumferentially spaced along the outer peripheral surface. The lugs have threaded bore holes, which receive screws for securing an interchangeable adapter plate adapted to connect the valve body to one of several types of pipe flange fittings. The adapter plate also overlies the outer periphery of the seat retainer and secures the seat retainer relative to the valve body. This adapter plate method of securing a retainer thus suffers from the same limitations as the radially extending lug designs discussed above.
In another prior art attempt to secure a seat retainer to a valve body while providing a full gasket face, threaded bores have been drilled radially inwardly through the valve body and into a central recess disposed about the flow passage. Set screws were then externally introduced through the threaded bores and extended into V-shaped grooves in the outer diameter of a retainer positioned in the recess. The set screws were then operative to hold the retainer in the recess. Although using a recessed retainer is highly advantageous, drilling holes through the valve body creates an additional leak path for the handled fluid.
In yet another prior art attempt to remedy the problems discussed above, a seat retainer is held in place on a valve body by a snap ring assembly. Snap ring assemblies have proved satisfactory for securing the retainer to the valve body during shipment. Further, snap ring assemblies are satisfactory in actual usage when the valve is sandwiched between two pipe flanges. So long as the retainer is compressingly interposed between the valve body and an adjacent pipe flange, there is no opportunity for the retainer to separate from the valve body.
However, it is common commercial practice to "bench test" valves prior to installing the valves in a fluid handling system. Bench testing is a common practice for calibrating an actuator for moving a valve. When a retainer is secured to a valve body by only a snap ring assembly, there is a significant danger that the force applied by the valve closure member during valve closing will apply a sufficient force to release the retainer from the valve body. In such an occurrence, the retainer is forcefully ejected from the valve body and becomes a highly dangerous projectile.
Moreover, difficulties are experienced in removing retainers secured by snap rings, such as for the replacement of a valve seat. It is common practice to position a screw driver between the valve seat and the retainer to pry a snap ring secured retainer from a valve body. However, such a procedure is likely to damage the seat, particularly if the seat is formed of a "soft" material.
Additionally, valves are occasionally used in dead end service at the end of a process line. In such a use, one face of the valve will not be abutted by an adjoining pipe flange. Thus, if the seat retainer is positioned on the exterior side of a dead end line and secured by only a snap ring assembly, the holding force of the snap ring assembly may be overcome and, the retainer may once again become a potentially dangerous projectile. Dislodgement of the retainer also frees the valve seat and results in gross leakage.